Lung surfactant is composed of a complex mixture of phospholipid, neutral lipid and protein. Surfactant is roughly 90% lipid and 10% protein with a lipid composition of 55% diphosphotidylcholine (DPPC), 25% phosphatidylcholine (PC), 12% phosphatidylglycerol (PG), 3.5% Phosphatidlyethanolamine (PE), sphingomyelin and phosphatidylinositol (PI).
Lung surfactant functions to reduce surface tension within the alveoli. It mediates transfer of oxygen and carbon dioxide, promotes alveolar expansion and covers the lung surfaces. Reduced surface tension permits the alveoli to be held open under less pressure. In addition, lung surfactant maintains alveolar expansion by varying surface tension with alveolar size (The Pathologic Basis of Disease, Robbins and Cotran eds. W. B. Saunders Co. New York, 1979). There are a number of medical therapies or regimes that would benefit from the use of surfactant supplements. For example, surfactant supplementation is beneficial for individuals with lung surfactant deficiencies. In addition, there are a variety of medical procedures requiring that fluids be added to the lung, for example, as a wash to remove endogenous or exogenous matter. The use of a biocompatible liquid for these applications would be advantageous. Routinely, balanced salt solutions or balanced salt solutions in combination with a given therapeutic agent are provided as an aspirate or as a lavage for patients with asthma, cystic fibrosis or bronchiectasis. While balanced saline is biocompatible, lavage procedures can remove endogenous lung surfactant. Further, lavage with such aqueous liquids may not permit adequate delivery of oxygen to the body. Therefore, it is contemplated that the use of substances having at least some of the functional properties of lung surfactant could decrease lung trauma and provide an improved wash fluid.
At present, surfactant supplements are used therapeutically when the amount of lung surfactant present is not sufficient to permit proper respiratory function. Surfactant supplementation is most commonly used in Respiratory Distress Syndrome (RDS), also known as hyaline membrane disease, when surfactant deficiencies compromise pulmonary function. While RDS is primarily a disease of newborn infants, an adult form of the disease, Adult Respiratory Distress Syndrome (ARDS), has many of the same characteristics as RDS, thus lending itself to similar therapies.
Adult respiratory distress occurs as a complication of shock-inducing trauma, infection, burn or direct lung damage. The pathology is observed histologically as diffuse damage to the alveolar wall, with hyaline membrane formation and capillary damage. Hyaline membrane formation, whether in ARDS or RDS, creates a barrier to gas exchange. Decreased oxygen produces a loss of lung epithelium yielding decreased surfactant production and foci of collapsed alveoli. This initiates a vicious cycle of hypoxia and lung damage.
RDS accounts for up to 5,000 infant deaths per year and affects up to 40,000 infants each year in the United States alone. While RDS can have a number of origins, the primary etiology is attributed to insufficient amounts of pulmonary surfactant. Those at greatest risk are infants born before the 36th week of gestation having premature lung development. Neonates born at less than 28 weeks of gestation have a 60-80% chance of developing RDS. The maturity of the fetal lung is assessed by the lecithin/sphingomyelin (L/S) ratio in the amniotic fluid. Clinical experience indicates that when the ratio approximates 2:1, the threat of RDS is small. In those neonates born from mothers with low L/S ratios, RDS becomes a life-threatening condition.
At birth, high inspiratory pressures are required to expand the lungs. With normal amounts of lung surfactant, the lungs retain up to 40% of the residual air volume after the first breath. With subsequent breaths, lower inspiratory pressures adequately aerate the lungs since the lungs now remain partially inflated. With low levels of surfactant, whether in infant or adult, the lungs are virtually devoid of air after each breath. The lungs collapse with each breath and the neonate must continue to work as hard for each successive breath as it did for its first. Thus, exogenous therapy is required to facilitate breathing and minimize lung damage.
Type II granular pneumocytes synthesize surfactant using one of two pathways dependent on the gestational age of the fetus. The pathway used until about the 35th week of pregnancy produces a surfactant that is more susceptible to hypoxia and acidosis than the mature pathway. A premature infant lacks sufficient mature surfactant necessary to breathe independently. Since the lungs mature rapidly at birth, therapy is often only required for three or four days. After this critical period the lung has matured sufficiently to give the neonate an excellent chance of recovery.
In adults, lung trauma can compromise surfactant production and interfere with oxygen exchange. Hemorrhage, infection, immune hypersensitivity reactions or the inhalation of irritants can injure the lung epithelium and endothelium. The loss of surfactant leads to foci of atelectasis. Tumors, mucous plugs or aneurysms can all induce atelectasis, and these patients could therefore all benefit from surfactant therapy.
In advanced cases of respiratory distress, whether in neonates or adults, the lungs are solid and airless. The alveoli are small and crumpled, but the proximal alveolar ducts and bronchi are overdistended. Hyaline membrane lines the alveolar ducts and scattered proximal alveoli. The membrane consists of protein-rich, fibrin-rich edema admixed with cellular debris.
The critical threat to life in respiratory distress is inadequate pulmonary exchange of oxygen and carbon dioxide resulting in metabolic acidosis. In infants this, together with the increased effort required to bring air into the lungs, produces a lethal combination resulting in overall mortality rates of 20-30%.
Optimally, surfactant supplements should be biologically compatible with the human lung. They should decrease the surface tension sufficiently within the alveoli, cover the lung surface easily and promote oxygen and carbon dioxide exchange.
Ventilation assistance is commonly used to provide sufficient oxygen to surfactant deficient patients. These ventilation regimes include continuous positive airway pressure, or continuous distending pressure procedures.
Recently, surfactant replacement therapy has been used either alone or in combination with ventilation therapy. Initial work with surfactant replacements used preparations of human lung surfactant obtained from lung lavage. The lavaged fluid is collected and the surfactant layer naturally separates from the saline wash. This layer is harvested and purified by gradient centrifugation. These preparations worked well as surfactant replacements for RDS and thus provided some of the original data to suggest that surfactant replacement was a necessary therapy. Supplies of human surfactant are limited and expensive, and therefore alternate sources of surfactant were investigated for use in replacement therapies.
The second generation of surfactant substitutes are purified preparations of bovine and porcine lung surfactant. Preparations of bovine lung surfactant have been administered to many surfactant deficient patients. Like human surfactant, bovine lung surfactant is difficult to prepare. Sources are few and availability is limited. Further, while the use of bovine lung surfactant in neonates does not present a problem immunologically, bovine surfactant applications in adults could immunologically sensitize patients to other bovine products.
To minimize the immunologic problems associated with the use of bovine lung surfactant, the harvested surfactant is further extracted with chloroform/methanol to purify the lipid component. This work led to the discovery that there are three major proteins (SP-A, SP-B and SP-C) associated with lung surfactant. All three are postulated to have some beneficial role in surfactant function. SP-A is hydrophobic and has some documented antibacterial activity. SP-B is most closely associated with traditional surfactant function. These proteins can be purified or cloned, expressed and added back to purified lipid preparations. However, these procedures are also time consuming. In addition, the use of purified animal-derived surfactant protein creates the same immunologic problems noted above.
Some of the functional domains within each of the surfactant proteins are now mapped and the individual lipid components of lung surfactant are being tested to determine if a semi-synthetic or synthetic product can be used effectively to replace purified endogenous surfactant. To this end, synthetic peptides of SP-B have been added to mixtures of DPPC and PG to create an artificial surfactant product.
An artificial surfactant would readily cover the lung surfaces and facilitate oxygen exchange. The surfactant would be sterilizable, amenable to large scale production and be relatively stable for convenient storage and physician convenience.
Fluorocarbons are fluorine substituted hydrocarbons that have been used in medical applications as imaging agents and as blood substitutes. U.S. Pat. No. 3,975,512 to Long uses fluorocarbons, including brominated perfluorocarbons, as a contrast enhancement medium in radiological imaging. Brominated fluorocarbons and other fluorocarbons are known to be safe, biocompatible substances when appropriately used in medical applications.
It is additionally known that oxygen, and gases in general, are highly soluble in some fluorocarbons. This characteristic has permitted investigators to develop emulsified fluorocarbons as blood substitutes. For a general discussion of the objectives of fluorocarbons as blood substitutes and a review of the efforts and problems in achieving these objectives see "Reassessment of Criteria for the Selection of Perfluorochemicals for Second-Generation Blood Substitutes: Analysis of Structure/Property Relationship" by Jean G. Reiss, Artificial Organs 8:34-56, 1984.
Oxygenatable fluorocarbons act as a solvent for oxygen. They dissolve oxygen at higher tensions and release this oxygen as the partial pressure decreases. Carbon dioxide is handled in a similar manner. Oxygenation of the fluorocarbon when used intravascularly occurs naturally through the lungs. For other applications, such as percutaneous transluminal coronary angioplasty, stroke therapy and organ preservation, the fluorocarbon can be oxygenated prior to use.
Liquid breathing has been demonstrated on several occasions. An animal may be submerged in an oxygenated fluorocarbon liquid and the lungs may be filled with fluorocarbon. Although the work of breathing is increased in these total submersion experiments, the animal can derive adequate oxygen for survival from breathing the fluorocarbon liquid.
Liquid breathing as a therapy presents significant problems. Liquid breathing in a hospital setting requires dedicated ventilation equipment capable of handling liquids. Moreover, oxygenation of the fluorocarbon being breathed must be accomplished separately. The capital costs associated with liquid breathing are considerable.
It is an object of the present invention to provide a method for treating lung surfactant deficiency through use of fluorocarbon liquids.
A further object of the invention is to provide a method for therapeutic use of fluorocarbon liquids in the lungs that does not require liquid-handling ventilation equipment. Instead, traditional gas ventilation equipment can be used.
Still a further object of the present invention is to apply pulmonary administration of fluorocarbon liquids to a wide range of diseases and medical conditions.
These and other objects of the invention are discussed in the detailed description of the invention that follows.